Genetics Chapter 6 linkage and Mapping in Eukaryotes

Mapping X-inked genes w/ pedigree analysis

"The grandfather method To understand this through in the genotypes and look at it. The biggest mistake is using the granddaughters for information. You have to use the grandsons

Three-point mapping strategy slide

8 different Parents, double single crossover We will have to find out what gene is in the middle? We will have to look at the double cross over classes compared to the parent which on switched that is the one in the middle Then write the haploid gamete phenotypes

Why is the # of tetratypes Halfed

?? You have to figure it out/ ask in class

Why isn' the number of non parental ditypes halved

??? You have to figure it out/ ask in class

Misc notes and terms

Crossing over does occur in mitosis (we have mitosis or somatic crossing over) Crossing over occurs in sister chromatids this is called sister chromatid exchange, it's just not interesting, this is when you have crossing over at two identical molecules There is an exception to law of segregation mendels first law Gene conversation and mieotic drive linkage is a way to cause excepts to the second law (independence assortment)

Linkage mapping is all about studying the effects of crossing over why are males not helpful

Crossing over happens in the male it won't change anything because all he can give is recessive, is the wild type hybrid individual. In fruit flies

What decided if we get a recombination event in a chromosome with linked genes

How far apart the genes are from one another is what decides it. If the genes are very far close we would not expect a recombination event to happen in the small place between the genes. If they are rather far apart then it will be way more likely we would end up with a bigger map distance.

Two-point mapping

How we use linkage to with these mapping crosses determine the distances between two genes.

Independent assortment

If the two mutants were not on the same chromosome and they were not linked we would exppect the 1:1:1:1 you expect and equal likelyhood becuase we essentially foil to get the proper gamates. This happens if the genes are not linked

What is the thing to remember with crossing over genes of 3-point, talk about this process and the rules that apply

If we have a crossover between the b, pr genes it is always the gene on the end that is crossing over. This means that we had a crossover that happened between b and pr on the left side. THE GENE THAT SWITCHES ON A SINGLE CROSSOVER IS ON THE END! We always want to take the parental type configurations. And then take those parental genes as the standard and see which genes switched and we can tell what two genes crossed over and it is always going to show the genes that switched closer to then end.

SDS

Is when you have a crossing over event. Second division segregation. This is when we do not segregate the alleles in the first meiosis division. It is not until meiosis 2 that the cells separate. The way this happens will be a 2:4:2 or a 2:2:2:2 configuration. It is based on which chromatids in the left to right experience a crossover.

What is the phenotypic ratio of bn+by det+ det X bn bn det det

It is a 1:1:1:1 and this is becomes of independent assotment. BUT the actual results is not 1:1:1:1

Why is it more likely to have a non parental crossover

It is already unlikely to hav a crossover event if these genes are linked and we are talking about ALL FOUR genes crossing over. So we can know if the genes are linked by looking at PD and NP ratios. If the two are similar in likely hood they are independent assortment not linked. If PD is highly likely in ratio you are dealing with linked.

Double Crossover what does this tell us what is the important rule here.

It is possible for cross over that happens on both. The gene that switches in the double crossovers is the one that is in the middle.

What gametes are produced by the triple recessive individual

Just all the recessive type alleles, they are going to be recessive this is all they are.

Haploid mapping

Look at all different asci, is it PD, NPD, or TT Compare the frequencies of PD and NPD IF they are linked then use the recombination frequency. The TT is halved

No parental ditypes

Now we only have two types and they do not match the original cross

Cis-configuration

On the same side, the two mutant alleles are present together linked together on the same chromsomes.

What sex has crossing over in fruit flies

Only in females

Coefficient of coincidence

Taking the observed double crossed over an dividing it by the expected double crossover.

Parental or non recombinant types

The actual results are not 1:1:1:1 we have 2 of the actual crossed banned and detahced these probability are WAY ABOVE almost 50/50. The frequencies these traits actually match the parents that were mated. These do not cross over.

How wil you know who is the parental types

The biggest numbers

Explain the spores forming in yeast

The centromere arrangement stays the same

Trans-configurations

The mutant alleles are not present on the same chromsome they are on different chromsomes. We can remember this do to the transport of the alleles. The

Recombination frequency

The percent of recombination, we would take the recombinate types add them together. Then divide this by the total number of indivduals

which gene switched

The purple is the one that switches we can sew how the B+ and The B in 388 and 367 this would be our switched because they are on the outside.

How do you know which one is the double crossover

The smallest numbers

FDS

There is no crossover event. This is the first division event . And it creates a 4:4 configuration within this octet

Recombinant Types

These are the genes that are linked close together on the chromsome, the only way you could get these kind of traits if crossover happens. This only happens if these two partiular genes are linked on the chromsome and they cross over together this is the only way it works.

What if the genes are really close together on the same chromosomes

They would be so close together that we would always see these two staying together and never see a recombination this is complete linkage.

Fluorescent in situ hydridzation

This causes hydridization of a DNA prop that have the sequence of the DNA region you are interested in and you can cause the hybridization to occur to that the prop will bind onto the DNA at which the base sequence is complementary and you can do imaging and see the glowing image and be able to say that this is where my prop went that is where the sequence I'm looking for is and that is useful.

Map units

This is the distance of our recmonations we take the recombination frequency and we times it by 100 then add the units

3-point mapping cross

This is used to save time. We show an example cross what the patterns will look like We take two genes that are 1 mutant and 1 wild type and we cross them We have to ask are they linked? We have to look at both possibilities and we see that in the bottom of the slide

Tetratypes

This is when you have the parental and non-parental ditypes.

Parental ditypes asci

This will have two types of and it will match the original cross, this is more likely

Somatic Cell Hybridization

We assign human genes to chromosome. We fuse two different cells together that are not the same species and it has two nuclei and it is a weird cell. When the cell starts to divide we actually loose some of hte human chromosomes

What if we have genes are are very far apart but still linked?

We could have some frequency of crossover events happens but we would expect a greater than 25% of the alleles if they are linked. But we would expect there to be some recombination depending on how far apart the genes are. The actual distance is the determining factor if a recombination event will happen.

How do we know if they are linked

We figure this out by looking at the gametes If they are not linked at all we would expect a 1:1 ratio If they are linked we would expect a half/ half mutant and wild

How do we know the somatic cell hybridization creates a loss of human chromosomes over time?

We look at a karyotype

What can we do for measuring in this crossover

We measure to the centromere. We just do A to B distance A to centromere distance and B to centromere distance.

Do we expect the double crossover to be frequent

We should expect the double cross overs to be very unlikely and therefore infrequent

Determining chromosome location

We will have to see which hybrid cell clones express the gene product. We have to look at the chromosome that are present in those clones the ones that produce the product and only those and once you answer that, that's it.

What are + super scripts used for

You have dominant and wild type alleles, and without the + you have he same letters but no + for recessive and mutant.

Interference

You need to calculate the 1-coefficient of coincidence. If the crossover happens at one area how much interference is going on to top the crossover.